Playing tricks on invasive rats may help keep birds from vanishing

Decoupling the scent of birds from a tasty meal may get the rats to ignore it.

Rats are among the most widely introduced invasive species in the world. Throughout history, they have hitched rides on boats, wagons, and trains, invading nearly every corner of the globe. Birds are a common prey for rats, and have been hit especially hard by the population increase and range expansion of these rodents; many avian species have become endangered, or even gone extinct, as a result of increased predation. Reintroducing birds is also a challenge, since resident rats quickly learn that the new species is a profitable source of food.

But now, two Australian scientists have identified a clever method that may limit predation by rats and increase bird survival. Their research, detailed in this week’s PNAS, offers a potential way to improve reintroduction success in areas where rodents are a problem.

Omnivorous predators such as rats often find new food sources by smell. They quickly investigate new scents, and if these odors are related to a recurring food reward—for example, if following a particular scent leads repeatedly to a nest full of plump baby birds—the scent and the reward become coupled in the rat’s mind. The researchers took this basic idea and reversed it: if they could decouplethe cue from the reward, would the rats learn to ignore the cue? In other words, if rats never learned that the scent of a nest proffered a tasty treat, would bird survival increase?

To determine whether this idea would work, the researchers conducted trials with wild rats in Sydney, Australia. They introduced domestic quail—a species that the rats didn’t already know as food—in two treatment groups. In the first treatment, the control group, quail nests with both real and artificial eggs were simply introduced into the rats’ habitat. Each day ten grams of “quail odor,” collected via feathers and feces from a nearby quail farm, was added to each nest to approximate realistic prey cues. Based on the number of eggs that were taken or damaged, the researchers could determine the rate of predation by rats (tooth marks in the artificial eggs helped researchers determine what species was responsible for the predation). This treatment illustrated how the reintroduction of a bird species might actually play out.

The second treatment was somewhat different; for seven days before the nests were introduced, the researchers applied quail odor across the habitat. Then, on the eighth day, the nests were put out, and predation was monitored as in the control group.

Incredibly, “survival” of the eggs was 62 percent higher in the second treatment group, where the rats were pre-exposed to the odor, than in the control group. Rats that were already familiar with the quail scent when the eggs were introduced raided nests significantly less often than did rats that were introduced to the odor and the nests simultaneously. This effect remained for seven days after the nests were added.

The scientists behind the study believe that if a novel odor is not reinforced with a reward immediately, when the rats’ motivation to investigate the new scent is strongest, the odor cue fades into the “olfactory background.” Once this occurs, it may be more difficult for the rats to learn that the odor is rewarding, even if there is later prey associated with it. By that point, familiar food may be more appealing to the predators.

The research has considerable implications for conservation, since decoupling a cue from its reward, at least initially, may result in more successful bird introductions. Since the study only lasted seven days, it’s impossible to know what could happen weeks, months, or years down the road; in time, rats may learn to link the cue with the prey. However, if reintroduced birds can be protected at their most vulnerable period, even for a short amount of time, the species may receive the bump it needs to thrive in its new habitat.

Kate Shaw Yoshida
Kate is a science writer for Ars Technica. She recently earned a dual Ph.D. in Zoology and Ecology, Evolutionary Biology and Behavior from Michigan State University, studying the social behavior of wild spotted hyenas. Emailkate.shaw@arstechnica.com//Twitter@KateYoshida

But in cases where familiar food sources may be scarce, wouldn't the rats just reinvestigate the background smells? This is definitely an interesting discovery, but I'm skeptical about any long term benefits.

Wonder if this "decoupling" could be applied to the human obesity problem?

2 million or so years of evolution predisposes us to liking salty, sweet and fatty foods. But I bet if those foods caused us to vomit, decoupling might be possible. It's kind of how like I am decoupled from drinking Southern Comfort.

To go along with the decoupling, it would be interesting to do a third test, put out only artificial eggs for the first week, but treat the eggs with something that makes the rats sick. After that week is up, add real eggs to the mix. My guess is that survival will be even higher in this third group.

The rats are invasive and reduced already healthy populations though, right? So getting the birds introduced isn't enough - for a method like this to be successful, you'd have to do it for every bird type, every year, forever. And that's assuming they don't figure the trick out eventually anyway. Hardly seems sustainable.

How would this last past one generation? Rats breed fairly often. I am by no means an expert on rats, but it seems that without repeated treatments the new generations would simply investigate a new odor and be rewarded. I agree with the other poster in that this hardly seems sustainable.

To go along with the decoupling, it would be interesting to do a third test, put out only artificial eggs for the first week, but treat the eggs with something that makes the rats sick. After that week is up, add real eggs to the mix. My guess is that survival will be even higher in this third group.

That's what I was thinking too; Wouldn't you want to decouple the scent and food source link completely by re-coupling it with a nasty surprise like a shock or something that makes them sick? And I wonder if the scent=bad response would last longer than the scent=nothing interesting response.

This idea is seems foolish to me. You can train the rats to not eat birds but after awhile in the wild they'll figure out that they can eat the birds just fine. As fast as rats multiply, it won't take long for this to happen. That's just how evolution works. I feel like Malcom from Jurassic Park... :-\

Regardless, if you want to train an animal you should be training birds not the rats. The birds are the ones that will receive the survival benefit.

This idea is seems foolish to me. You can train the rats to not eat birds but after awhile in the wild they'll figure out that they can eat the birds just fine. As fast as rats multiply, it won't take long for this to happen. That's just how evolution works. I feel like Malcom from Jurassic Park... :-\

If it can give reintroduced species a head start, that may make the difference in helping them become established when combined with other measures like pest eradication and control.

Quote:

Regardless, if you want to train an animal you should be training birds not the rats. The birds are the ones that will receive the survival benefit.

Does this mean that the test ended after 7 days or things went back to the same rate as the control after 7 days? If its the latter then there doesn't seem to be much hope of this having any real effect, though it is an interesting idea.

Does this mean that the test ended after 7 days or things went back to the same rate as the control after 7 days? If its the latter then there doesn't seem to be much hope of this having any real effect, though it is an interesting idea.

Sorry, let me clarify: the researchers only monitored the nests for seven days after introducing the nests, and the effect persisted until the test ended.

By pre treating the ground you "teach" the rats that there's nothing special about the scent but surely after a certain amount of time the rats would learn that the smell would be associated with food simply by coming across nests and feeding on eggs or birds?

I think the idea is to give the birds a head start so they can multiply before the rats start eating the eggs. Its ok for the rats to eat the eggs, it keeps the population down, but when the population is already low because you just put them there it isnt.

This idea is seems foolish to me. You can train the rats to not eat birds but after awhile in the wild they'll figure out that they can eat the birds just fine. As fast as rats multiply, it won't take long for this to happen. That's just how evolution works. I feel like Malcom from Jurassic Park... :-\

If it can give reintroduced species a head start, that may make the difference in helping them become established when combined with other measures like pest eradication and control.

Quote:

Regardless, if you want to train an animal you should be training birds not the rats. The birds are the ones that will receive the survival benefit.

I have no idea, but obviously it's the bird and not the rats that has the incentive to change. You teach the rat and it will just unlearn it. If you teach the bird it's survival will depend on learning. So either you come up with something to teach birds your find another strategy.

As for the head start, the birds already had that and they became food. It will go even worse the next time around because it's the rats that have the head start with the massive population advantage.